Patent GB-1244297-A (GB)
Title: A data communication system
IPC: H04L12/423
Filed: 1970-04-21
Abstract: 1,244,297. Electric selective signalling. INTERNATIONAL BUSINESS MACHINES CORP. 21 April, 1970 [16 May, 1969], No. 18879/70. Heading G4H. A data transmission system consists of a central station and a plurality of area stations connected in series in a loop, each area station having means, operable in response to a predetermined signal from the central station, and when the area station has data to send, to transmit its address to the central station, the central station being able to select any of the responding stations. In the form of Fig. 1 the central station 2 sends data out on to the loop 4 and receives it from the loop via modem 3. Terminal A has a control unit 12 connected to the loop via mode m 11. It may consist of a number of sub-stations 13, 14, 15 &c. Each terminal has a shift-register 20, Fig. 2, able to hold one character. It may be connected in series with the loop 4, or not, by two-position switch 24. When at rest the station 2 sends a first character A which may be used to synchronize the area stations. When the central station wants to know the identities of area stations having data to send it sends a second character B. At this time all the switches are in position 25 so that character B passes round the loop and is received simultaneously in the shift registers of all the terminals. The character B is detected by a decoder 21 and the result passed to control device 23. If there is data to be transmitted terminal is readied: its identity is entered into the shift register in parallel and its switch 24 is changed to position 26. If there is no data these operations do not take place. After sending character B the central station 2 continues sending character A. The first readied terminal on the loop receives character A in the shift register, the identity character being displaced on to the line and loaded into the shift register of the next readied terminal, displacing the identity character therein. This moves to the next readied terminal and so on until the last readied terminal transmit the identity character to the central station 2. When the next character A is sent it is received in the second readied terminal the identity characters of all the readied terminals moving one step towards the central station, the next to last being received at the central station. In this way all the identities are passed to the central station, the central station finally receiving the character A. The central station is able to select for data transmission any one of the readied stations by sending a third character C followed by the identity character of the selected character. This causes the switch 24 of the selected terminal to move to position 26 and the shift register to be loaded from store 22 with the data to be sent. The data is caused to be sent by the continuous transmission of the character A from the central station. In another form (Fig. 4, not shown) there is one register to receive characters coming from the central station and for passing them on again to the loop unchanged and a second register for transmitting characters from the terminal to the loop. The switch has three positions to select the incoming or outgoing registers, or the by-pass. In a further embodiment (Fig. 6, not shown) there is no by-pass or switch, the shift register being connected directly in the loop. Characters from the central station are transferred from one register to the next. When the central station sends character B and the terminal has data to send the identity character is entered in the register to replace the character A following the character B.